Generally the practice in instrumentation is to provide a rigid carcass as an instrument case in which instrument components, for example circuit boards, are mounted. Typically such a case has internal bracing members which maintina rigidity whilst side, front, rear top and bottom panels are removable; this is wasteful in that when the case is completely assembled, such members are structurally redundant, wasting speace and adding to weight.
In the design of electronic instruments, for example spectrum analysers, data loggers, vibration controllers and the like, ther is a trend towards modularity, i.e., to instruments which can be configured, reconfigured and serviced by the exchange of modules. An attractive way of providing modularity is to provide a mother-board which is adapted to receive and interconnect a plurality of circuit boards via plugs and sockets.
This practice has led to the wide adoption of card frame structures wherein a plurality of circuit boards are supported substantially parallel by slots or rails in a free standing frame having a mother board mounted within in a plane normal to the boards. It is important that reliable connection between the boards and the mother-board is made and to this end mother-board is generally rigidly supported upon a structural member. Typically a card frame is made in metal in channel section spars bolted together to provide a rigid free standing structure. The frame may then be cased by simply bolting side, front and rear facing panels to the structure. Card frames are ideally suited to some applications, for example static digital computers wherein a plurality of physically similar boards may be plugged into a mother board to give a complete working arrangement, non-computer components generally being arranged as separate peripherals.
In the art of electronic instrumentation, there is increasing use of digital electronics and computer technology making modularity attractive. Unfortunately, card frames are less than ideally suited to instrumentation since instruments usually include a number of interface components, such as displays, keyboards, printers, plotters and disk or tape drives, which are not adapted to card mounting. As previously described, the practice in instrumentation is to provide a rigid carcass, the carcass providing the instrument case, in which such components are rigidly mounted together with circuit boards. To mount a card frame within such a carcass is wasteful in that one rigid structure is mounted inside another, inevitably leading to increased weight and loss of space, which is undesirable in a self-contained instrument, particularly a portable instrument.
An additional disadvantage of a card frame in relation to electronic instruments is that it is generally not possible easily to exchange a mother-board, it being often embedded within the frame structure. It is more likely in instrumentation that there will be differences in mother-boards between models which may have differing interfaces than in, for example, digital computers relying on a common processor with differing peripherals.